MM-206 is an inhibitor of STAT3 (IC50 = 1.16 μM in an SPR-based competitive assay). It inhibits STAT3 phosphorylation induced by granulocyte colony-stimulating factor (G-CSF) in HL-60, Kasumi-1, and MOLM-13 acute myeloid leukemia (AML) cells (IC50s = 0.8, 1.9, and 1.1 μM, respectively). MM-206 induces apoptosis more potently in AML cell lines and patient-derived primary AML tumor cells than in acute lymphoblastic leukemia cells, which do not have upregulated STAT3 activity. It also slows disease progression and improves survival in an AML mouse xenograft model when administered at a dose of 30 mg/kg per day.